Conventionally, as an infrared detector, either a thermal type or photo type has been used. Of these types, for measurement of temperatures in living space, the thermal type detector is used which is of relatively high sensitivity in a wavelength range of 5 .mu.m or more and is inexpensive. Typically available as the thermal type detector is a pyroelectric type element which is used for an electronic oven, crime prevention and the like. In addition, a thermistor bolometer, a thermopile and the like are available. All of these devices or elements, however, provide analog outputs and have unsatisfactory noise-proof properties. Therefore, the advent of an infrared detector having digital outputs has been desired.
With the development of microcomputers, on the other hand, the advent of an element has been desired which is capable of directly inputting digital signals to a microcomputer.